For cellular systems, current techniques for determining a location of a mobile device include using a Global Positioning System (GPS), Assisted GPS (A-GPS), Time Difference of Arrival (TDOA), Enhanced Observed Time Difference (E-OTD), Radio Fingerprinting, Cell Global Identity, and Enhanced Cell ID.
GPS is a space-based satellite constellation that provides constant, global geopositioning to end users. A user's GPS device processes signals from three or more GPS satellites, and by way of trilateration provides latitude and longitude data. GPS may be global, but it is not ubiquitous because foliage, building edifices, and bridges block GPS signals. The absence of GPS signals in places such as the urban core is problematic because high value applications for reliable positioning abound, yet no reliable solution exists. The urban core in major cities is also known as the “urban canyon,” because skyscrapers on both sides of the streets block GPS signals. Another common problem is multipath distortion in which signals bounce off many surfaces before reaching the user, resulting in vastly inaccurate positioning data.
In addition to providing navigation within the urban core, GPS is widely used to aid navigation worldwide. GPS was originally designed as a military system and has had recent broad adoption for civil and civilian uses due to its inherent accuracy and a strong history of reliable performance. Several other entities, including the European Union, Russia, India, China, and Japan have satellite-based Position, Navigation, and Timing deployed or in the process of deploying. Collectively, the use of satellite-based navigation services is referred to a Global Navigation Satellite Systems (GNSS). GPS, however, remains the de facto system of choice, primarily because of its maturity, with over fifteen years of reliable free service. GPS acceptance has also been enhanced by open interface control documentation (ICD), which allows receiver manufacturers to confidently design systems against a reliable standard.
Because GNSS systems are satellite-based, however, there is a danger that either natural or human threats to these systems could make them vulnerable to outages. Further, other locating systems such as E-OTD may not provide a desired level of accuracy or may be overly expensive.